Allow addons to inject middleware into testem

This PR depends upon https://github.com/airportyh/testem/pull/410 being
accepted

Primarily the use case is with any addon that injects a middleware into
the ember server's, this will allow addon authors to also expose that
middleware through `ember test` if/when noted PR above is accepted by
@airportyh
Merge pull request #2128 from bcardarella/bc-support-testem-middleware

Allow addons to inject middleware into testem